This invention relates to an epoxy based adhesive containing a toughening agent and a method of applying such adhesive using a streaming process.
Epoxy resin based adhesives are used to bond a variety of different substrates together. In the automotive industry, epoxy resin adhesives are used to bond certain parts together, and are known as structural adhesives. A structural adhesive is an adhesive which bonds parts of the body structure of an automobile together. The problem with epoxy resins as used in adhesive compositions is that the epoxy resins are somewhat brittle and subject to fracturing when impacted. This tendency to fracture can be reduced by the addition of toughening agents. The problem with the use of toughening agents is that such toughening agents tend to increase the viscosity of the composition and the increased viscosity limits the method and speed of application. Mülhaupt, U.S. Pat. No. 5,278,257 discloses an epoxy resin containing (a) copolymer based on at least one 1,3-diene and at least one polar, ethylenically unsaturated comonomer and (b) a phenol-terminated polyurethane, polyurea or polyurea urethane. The adhesives disclosed in Mülhaupt are excellent structural adhesives. Because these adhesives are very viscous, they are not useful in some high-volume applications which require high application speeds.
In particular these materials are applied generally as an extruded bead directly on the surface and have a viscosity of from about 150 to about 600 Pa·s. measured at 45° C. In another application these adhesives are applied using swirl techniques at a viscosity of about 100 Pa·s. The process of applying an adhesive using an extruded bead is too slow for many high speed applications. The swirl process is a faster application but is not ideal for high-volume structural applications.
What is needed is a process for applying structural adhesives based on epoxy resins wherein the adhesive can be applied fast and in high-volume.